Provide an upgradable readers-writer lock

[PragmaTwice]

Do you think if it's a good idea?

See references about "upgradable":

• https://uxlfoundation.github.io/oneTBB/main/tbb_userguide/UpgradeDowngrade.html
• https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock#Upgradable_RW_lock

[tisonkun]

This sounds reasonable. I'd prefer to see an overall Rust API design since the main challenge in Async Rust primitives is proper API semantic.

[mhambre]

I know parking_lot supports this so there's definitely some desire out there to have a feature for it. Longer term we could look into it and follow similar semantics since their lock_api's RawRwLockUpgrade as an idea for extending existing core types is very clean (e.g. trait extension of RwLock or make it into a 'Raw' type so that we can further extend with things like different priority structures). This is something that could be applied to other synchronization methods as well. For a quicker turnaround we could build it as a separate type? Furthermore we can derive lessons from async-lock as it's also runtime agnostic and can build it from the existing asyncband primitives.

[tisonkun]

For a quicker turnaround we could build it as a separate type?

This is possible. We may have a draft implementation and see.